Marketed products commonly require some type of marking on the product for commercial, regulatory, cosmetic or functional purposes. Desirable attributes for marking include consistent appearance, durability, and ease of application. Appearance refers to the ability to reliably and repeatably render a mark with a selected shape, color and optical density. Durability is the quality of remaining unchanged in spite of abrasion to the marked surface. Ease of application refers to the cost in materials, time and resources of producing a mark including programmability. Programmability refers to the ability to program the marking device with a new pattern to be marked by changing software as opposed to changing hardware such as screens or masks.
Stainless steel, which is strong, and has a durable surface finish, has many applications in industrial and commercial goods. Many articles manufactured out of metals such these as are in need of permanent, visible, commercially desirable marking. Stainless steel is an exemplary material that has such needs. Metals such as stainless steel which resist corrosion can be marked in this fashion. Marking stainless steel with laser pulses produced by a laser processing system can make durable marks quickly at extremely low cost per mark in a programmable fashion.
Creating color changes on the surface of stainless steel with laser pulses has been reported in the literature. One mechanism which has been put forth to explain the change in optical density or color of metallic surfaces is the creation of laser-induced periodic surface structures (LIPSS). The article “Colorizing metals with femtosecond laser pulses” by A. Y. Vorobyev and Chunlei Guo, Applied Physics Letters 92, (041914) 2008, pp 41914-1 to 141914-3 describes various colors which may be created on metals using femtosecond laser pulses. This article describes making black or gray marks on metal and creating a gold color on metal. Some other colors are mentioned but no further description is made. LIPSS is the only explanation offered for the creation of marks on metallic surfaces. Further, only laser pulses having temporal pulse widths of 65 femtoseconds are taught or suggested to create these structures.
Two articles discuss using picosecond laser pulses to create surface changes on semiconductor materials and metals. The articles SURFACE RIPPLES ON SILICON AND GALLIUM ARSENIDE UNDER PICOSECOND LASER ILLUMINATION, authors P.M. Fauchet and A. E. Siegman, Appl. Phys. Lett. 40(9), 1 May 1982, pp 824-826, and GRADUAL SURFACE TRANSITIONS ON SEMICONDUCTORS INDUCED BY MULTIPLE PICOSECOND LASER PULSES, author P. M. Fauchet, Physics Letters, Vol. 93A, No. 3, 3 Jan. 1983 both describe in detail the changes that occur on semiconductor and metal surfaces when subject to infrared and visible wavelength picosecond laser pulses. These articles describe how ripples form on the surface of these materials but do not discuss how the appearance of the material changes as a result of laser interaction.
Another problem with reliably and repeatably producing marks with desired color and optical density in stainless steel is that the energy required to create very dark marks with readily available nanosecond pulse width solid state lasers is enough to cause damage to the metal, an undesirable result. “Darkness” or “lightness” or color names are relative terms. A standard method of quantifying color is by reference to the CIE system of colorimetry. This system is described in “CIE Fundamentals for Color Measurements”, Ohno, Y., IS&T NIP16 Conf, Vancouver, Conn., Oct. 16-20, 2000, pp 540-545. In this system of measurement, achieving a commercially desirable black mark requires parameters less than or equal to L*=20, a*=+/−2, and b*=+/−2 on the CIE chromaticity scale. This results in a neutral colored black mark with no visible grayness or coloration.
What is desired but undisclosed by the art is a reliable and repeatable method of making commercially desirable black marks on stainless steel that does not require an expensive femtosecond laser or ablate the surface of the metal. What is needed then is a method for reliably and repeatably creating marks having a desired optical density on stainless steel using a lower cost laser, without causing undesired damage to the surface or requiring cleaning prior to anodization.